1. Field of the Invention
This invention relates generally to linear antennas utilized for radio broadcast and reception, specifically to vertical and horizontal single and multiband antennas, horizontal arrays, and shortened antennas for mobile use. The antenna is especially useful for multiband operation on the 80/75 meter, 40 meter, 20 meter, 15 meter, and 10 meter bands.
2. Description of the Prior Art
The fundamental linear antenna is the dipole, which may be oriented horizontally or vertically. In its most basic configuration, it consists of two colinear conducting wires (each of length equal to one-quarter of the operative wavelength--i.e.--"1/4"). The antenna is connected at its central point to a source of alternating current oscillating in the radio frequency range (the "rf source"), its two wires being connected at that point to opposite poles of said rf source via an appropriate transmission line. The length of each of the aforesaid wires (1/4 .lambda.) as well as the resultant overall length of the dipole (1/2 .lambda.) has been established to properly phase the current in each with respect to the other.
To conserve on overall height, the lower half of the vertical dipole ("vertical") is commonly discarded and replaced by the ground or Earth's surface. In this situation the ground surface acts as an imaging surface plane. The reflective characteristics of this plane create the replacement for the lower half of the vertical radiator, thereby reducing the total height from 1/2 .lambda. to 1/4 .lambda.. However, in most locations, the Earth's surface is a poor conductor. Thus, it is typically necessary to enhance soil conductivity by placing a wire mesh or a number of radially oriented wires ("radials") beneath the vertical, on or below the surface of the ground. The major portion of the following descriptions addresses the vertical antenna configuration; however, as will be seen, the invention is not limited to verticals, but is equally applicable to horizontal antennas ("horizontals").
The typical vertical, as described above, receives current at its base, one current element being attached to the vertically oriented wire, and one being attached to the radially oriented wires. Current flow is inward on the radials when current flow on the vertically oriented wire is upward, and outward on the radials when current flow on the vertically oriented wire is downward. In order to effect the most efficient transfer of power from the transmission line to the antenna, the impedance of each must be identical. The characteristic impedance of the transmission line is a function of conductor diameter, conductor spacing, and the material which is used to separate the wires. The impedance of the antenna, commonly referred to as "antenna resistance," is actually a measure of its power. The dipole consumes power, but rather than producing heat, it radiates electromagnetic energy.
Although feasible, transmission lines with a multiplicity of different impedances are not available. 52, 75 and 90 ohm lines are the most readily available; however, as most rf sources are 52 ohm devices, 52 ohm transmission line is the most common. It is, therefore, desirable that all antennas have a 52 ohm antenna resistance in order to effect a matched, maximum power transfer. It is also desirable to utilize a single antenna for several wavelengths. Currently, in order to utilize an antenna for more than one wavelength, one of the following methods is employed to adjust the height to 1/4 .lambda.: (a) trap isolation; (b) multiple antennas attached to a single structure; and (c) remote controlled motorized tuning assemblies located at the base of a single mast. None of these methods has, however, proved totally satisfactory.
The trap multiband vertical contains a number of hi-impedance, parallel resonant, "traps" inserted in series at the requisite heights on the vertically oriented wire. Each trap effectively disconnects that portion of the antenna above the trap. Amateur radio operators utilize five major wavelengths: 80/75 meters (3.5 to 4 mhz); 40 meters (7 to 7.3 mhz); 20 meters (14 to 14.4 mhz); 15 meters (21 to 21.5 mhz); and 10 meters (28 to 29 mhz). Thus, in a typical antenna operating at these wavelengths, the 10 meter trap is located eight (8) feet above the base (i.e.--one-quarter (1/4) of 10 meters, the operative wavelength), and disconnects that portion of the antenna above the trap. The 8 feet utilized is the portion of the antenna closest to the ground with the poorest visibility over nearby objects. However, the lowest 8 feet must be utilized because the antenna is base excited. When a longer wavelength is selected, less of the antenna is discarded, the entire antenna height finally being utilized when the longest wavelength is broadcast.
On the lowest band all the previous traps become loading coils since they are no longer resonant at the lowest frequency. These loading coils force antenna height to be decreased to compensate for its longer length electrically. The shortened antenna then presents a very low antenna resistance, typically in a range from 6 to 10 ohms. An external device like a transformer must now be added to transform this resistance up to 52 ohms. The transformation network required to handle the entire antenna at its various operating wavelengths adds to loss of antenna power. It also becomes very complicated due to the fact that each decrease in wavelength involves another trap and an increased antenna resistance. Under these conditions it is nearly impossible to match antenna resistance and transmission line impedance over all five bands.
Multiple antennas on a single structure and antennas featuring motorized tuning assemblies present two alternate methods of adjusting antenna height. The multiple antenna utilizes a vertical tower constructed such that it has antennas of various heights mounted thereon. As with the trap antenna, it receives current at its base and the total height of the structure is not utilized on each band. However, in comparison to the trap antenna, antenna radiation resistance remains more constant at varying wavelengths. Nonetheless, some variation appears due to the effect one antenna has on another when the two are in close proximity.
The motorized tuning antenna employs a remotely controlled (motorized) assembly that is generally placed at the base of the antenna mast. The tuning antenna contains a variety of rotary, inductive and capacitive assembles that can be remotely controlled via internal motors and gears. Units of this type are expensive because they are complex and require great care in design and fabrication to avoid malfunction due to external conditions such as extremes of temperature, corrosion from salt air, water vapor penetration and destruction from lightning. Further, the units can result in loss of power due to the extreme range of transformation required when a single mast must be matched to 52 ohms.